WASHINGTON — The lawyers of two Texas death row inmates asked Attorney General Eric Holder Wednesday to open an investigation on the state's alleged illegal purchases of lethal injection drugs.

In a letter made public by the lawyers, they said they had uncovered official documents through a Freedom of Information Act request that "leads us to believe that the Texas Department of Criminal Justice has violated the federal Controlled Substances Act."

According to the lawyers, the state has purchased for the past 25 years controlled substances used in lethal injections by using a Drug Enforcement Administration registration number for the the Huntsville Unit Hospital, which has been closed since 1983.

The Texas execution chamber is located in the same city.

"As a result, we believe that TDCJ is unlawfully in possession of and unlawfully dispensing controlled substances," the lawyers said.

Prison authorities did not immediately respond to AFP's requests for comment.

Attorneys for condemned convicts Cleve Foster and Humberto Leal noted that the products used by correctional authorities to carry out executions were "neither kept by a pharmacy, hospital or clinic, nor dispensed by an authorized practitioner through a prescription or otherwise."

Rather, they were kept inside the state's Huntsville unit where executions take place by TDCJ staff, rather than officials legally authorized to be in possession of the drugs.

"Given these potential violations of federal law and the resulting impact on the legality of the executions imminently contemplated, we respectfully urge you to direct appropriate agencies within your department to conduct a prompt and thorough investigation of these issues," the lawyers wrote.

Their letter was also addressed to Texas Department of Public Safety chief Steve McCraw.

"Federal and state laws governing the purchase, possession and transfer of controlled substances exist to guard against diversion, and ensure the efficacy and lawful use of these drugs," said Maurie Levin, an Austin attorney who represents Foster.

"For TDCJ to have misrepresented for 25 years the information the DEA relies upon to assess the legitimacy of these drugs reflects a profound disregard for protocol and the law."

On Tuesday, Levin had asked a court to stop the executions of Foster, who is to be executed April 5, and Leal, whose execution has been scheduled for July 7, claiming Texas violated procedure when it switched drugs to be used for lethal injections.

Texas, which executes more inmates than any other US state, announced March 16 that it will replace thiopental with pentobarbital, an anesthetic commonly used to euthanize animals.

Sodium thiopental is no longer manufactured in the United States and existing stocks are expiring.

Texas made the switch because its sodium thiopental expires at the end of March, according to Michelle Lyons, public information director for the Texas Department of Criminal Justice.

But lawyers for the inmates argue that Texas failed to follow administrative procedure requiring public notification of such changes.